There should be no problem, but my experience is that you cannot set the 7390 up to work in one mode then change the internet / account settings to work in a different mode, such as to connect through the BT-provided modem: it simply doesn't work. What you have to do is go through the wizard to set it all up from scratch.
I didn't realise this, and set everything up, phones, VOIP, voicemail etc and tested it using my existing ADSL setup, not using it to create the connection (it is a rubbish ADSL2 modem). Then when the FTTC was in place I tried to get the 7390 to connect through the FTTC modem, it just didn't work - PPPoE timeouts.
Started again from scratch, with a factory reset and using the wizard and set it up to connect using my FTTC username and password through the BT FTTC modem and it worked straight away.